Introduce Backend trait to allow different RPC (or other) backends to be implemented (#1126)

* WIP backend trait

* WIP converting higher level stuff to using Backend impl

* more implementing new backend trait, mainly storage focused

* Get core code compiling with new backend bits

* subxt crate checks passing

* fix tests

* cargo fmt

* clippy/fixes

* merging and other fixes

* fix test

* fix lightclient code

* Fix some broken doc links

* another book link fix

* fix broken test when moving default_rpc_client

* fix dry_run test

* fix more tests; lightclient and wasm

* fix wasm tests

* fix some doc examples

* use next() instead of next_item()

* missing next_item() -> next()s

* move legacy RPc methods to LegacyRpcMethods type to host generic param instead of RpcClient

* standardise on all RpcClient types prefixed with Rpc, and 'raw' trait types prefixed with RawRpc so it's less ocnfusing which is which

* rename fixes

* doc fixes

* Add back system_dryRun RPC method and rename tx.dry_run() to tx.validate(), to signal that the calls are different

* Add a test that we return the correct extrinsic hash from submit()

* add TransactionValid details back, and protect against out of range bytes

* add test for decoding transaction validation from empty bytes

* fix clippy warning

subxt/src/config/mod.rs

@@ -26,10 +26,11 @@ pub use signed_extensions::SignedExtension;
 pub use substrate::{SubstrateConfig, SubstrateExtrinsicParams, SubstrateExtrinsicParamsBuilder};
 
 /// Runtime types.
-// Note: the 'static bound isn't strictly required, but currently deriving TypeInfo
-// automatically applies a 'static bound to all generic types (including this one),
-// and so until that is resolved, we'll keep the (easy to satisfy) constraint here.
-pub trait Config: Sized + 'static {
+// Note: the `Send + Sync + 'static` bound isn't strictly required, but currently deriving
+// TypeInfo automatically applies a 'static bound to all generic types (including this one),
+// And we want the compiler to infer `Send` and `Sync` OK for things which have `T: Config`
+// rather than having to `unsafe impl` them ourselves.
+pub trait Config: Sized + Send + Sync + 'static {
     /// The output of the `Hasher` function.
     type Hash: Debug
         + Copy